Coherent imaging with reduced speckle

ABSTRACT

An improved technique for imaging a multiplicity of planes of non-diffuse objects, including non-diffuse transparencies, with coherent illumination which utilizes a diffusion structure having a periodic phase thereacross.

[ Aug. 28, 1973 I COHERENT IMAGING WITH REDUCED SPECKLE Inventor: EmmettN. Leith, Plymouth,

Mich.

Assignee: Battelle Development Corporation, Columbus, Ohio Filed: Jan.19, 1971 Appl. No.: 107,853

Related US. Application Data [63] Continuation of Sen No. 820,879, May1, I969,

3,545,835 l2/l970 Leith et al. 350/3.5 3,421,809 l/l969 Lohmann 350/162SF OTHER PUBLICATIONS Gerritsen et 31., IEEE Jour. of QuantumElectronics, Vol. QE-4, No. 5, May, 1968, p. 376.

Upatnieks, Applied Optics, Vol. 6, No. l 1, Nov., 1967, pp. 1,905-l,9l0.

Primary Examiner-Ronald L. Wibert Assistant Examiner-Ronald .l. SternAttorney-Woodcock, Washburn, Kurtz & Mackiewicz abandoned.

. 57 ABSTRACT 350/162 350/162 /35 An improved technique for imaging amultiplicity of [51] Int. Cl. G02b /02 planes f non-diffuse objects,including non-diffuse Field of Search 162 162 transparencies, withcoherent illumination which uti- 240/1 R lizes a difiusion structurehaving a periodic phase thereacross. [56] References Cited CM 9 D i FlUNITED STATES PATENTS gum 3,523,054 8/1970 Heflinger et al. 350/353,539,24l ll/l970 Upatnieks 350/35 DIFFUSING OBJECT IMAGING IMAGESTRUCTURE Z3 SPACE SYSTEM PLANE I COHERENT 1 I LIGHT I U I I RCE h 7 IPatented Aug. 28, 1973 2 Sheets-Sheet 1 TMAGING IMAGE SYSTEM PLANEOBJECT SPACE mFFUsmG STRUCTURE Z3 COHERENT LIGHT SOURCE COHERENT LIGHTFly. 2

DlFFUSANG STRUCTURE L'g. 3A

Patented Aug. 28, 1973 3,754,814

2 Sheets-Sheet 2 l V w COHERENT LIGHT SOURCE FLLQ'. 4B

R E LATIVE PHASE Fig. 5 Fig. 5A

C OHERENT IMAGING WITH REDUCED SPECKLE This application is acontinuation of parent application Ser. No. 820,879 filed May 1, 1969,now abandoned.

I BACKGROUND OF THE INVENTION This invention relates generally tooptical techniques of forming an image of an object and morespecifically relates to imaging non-diffuse objects with coherent lightillumination.

Since the use of a laser as a source of coherent light has becomepractical, imaging with coherent light where advantageous has becomecommon. One application of coherent imaging is in the field ofmicroscopy where a three-dimensional object transparency to be examined,such as a biological specimen, is illuminated with coherent light andviewed with an optical system which magnifies the information containedin the transparency many times. Another application of coherent image isin the field of holography wherein a hologram is constructed which iscapable of reconstructing at a later time the wavefront initiallyemanating from the object. In these and other applications of coherentlight illumination, it is highly desirable that an object be illuminatedand viewed without unwanted information in the form of noise beingintroduced into an object image by the illuminating or viewing system.

The use of coherent light to illuminate a non-diffuse object such as atransparency presents a problem not encountered in ordinary incoherentimaging of such objects. A major problem encountered is the formation ofdark rings in the object image resulting from such things as dust ordirt upon the object or on an element of the optical system used in theimaging process. These dark rings are much broader than the resolutioncapabilities of the system and will draw the observers attention awayfrom the object image presented and may be so severe that a portion ofthe object information is obliterated. Such rings are formed frominterference between light which is scattered by the dust andunscattered light at the image plane.

To eliminate or reduce the formation of these undesirable dark ringswhich are formed along with the image of the object, a diffusion platesuch as ground glass is placed in the coherent illumination beam priorto its striking the object. The diffusion plate is chosen to vary thephase across the object-illuminating beam. Illumination with such a beamprevents the aforementioned formation of broad dark rings (usuallycalled noise) in an image since there are no longer two highly uniformwavefronts interfering at the image plane. These dark rings are ineffect broken into many dark patches, each patch being reduced in sizeto the resolution capability of the system. The substitution of tinypatches for the broad dark bands greatly enhances the appearance of theimage and does not obscure desirable detail. The use of a diffusingplate in a coherent light transparency imaging system is more fullyexplained by Leith and Upatnieks in the Journal of the Optical Societyof America, Nov., 1964, beginning at page 1,295. The use of a diffusionplate for coherent illumination in holography is also'described by Leithand Upatnieks in copending application Ser. No. 361,977 filed Apr. 23,1964, now US. Pat. No. 3,506,327 issued Apr. 14, 1970.

Although the use of a diffusion medium such. as a ground glass plate inthe object illuminating coherent light beam efiectively reducesinterference ring noise in the image ofa non-diffuse object, anothertype of noise is introduced into the system by the ground glass plate.This added noise is a discernible grainy background to an image of anobject which results from the existence of non-uniform intensity acrossthe coherent beam illuminating the object. This type of noise issometimes referred to as speckle".

This grainy noise can be efi'ectively eliminated by illuminating thenon-diffuse object with a coherent light beam having a uniform intensitythereacross. This is reported by Uptanieks in Applied Optics, November,1967 beginning at page 1,905, and also in a copending application byUpatnieks, Ser. No. 638,031, filed May 12, 1967, now US. Pat. No.3,539,241 issued Nov. 10, 1970. Upatnieks describes accomplishing thisresult by using a pure phase plate which is characterized by randomphase shifting properties across the face of the diffuser but withuniform amplitude transmission thereacross. However, such a diffuser isuseful only to illuminate a two dimensional non-diffuse transparency asan object and furthermore must be placed immediately against such atransparency or imaged thereon in order to illuminate the transparencywith uniform intensity. Therefore, such a random phase plate difi'userdoes not eliminate the grainy noise problem of coherent illuminationwhen a three-dimensional object is desired to be illuminated in aplurality of planes with uniform intensity. Furthermore, even when onlya two dimensional transparency is desired to be illuminated with uniformintensity, it-is generally inconvenient to be limited in the positionthat the transparency must take relative to the diffuser for uniformillumination thereof.

Therefore, it is a primary object of this invention to provide animproved technique of illuminating nondiffuse three-dimensional objectswith coherent light for imaging thereof.

It is also an object of this invention to image a plane of a non-diffusetransparency removed a distance downstream from a diffusing structure ina manner to substantially minimize image noise in the form ofinterference rings and speckle.

It is a further object of this invention to provide a coherent lightbeam having at any plane along a substantial length a varying phasethereacross and either uniform intensity or an intensity variationthereacross that may be utilized for illuminating an object for imaging.

SUMMARY OF THE INVENTION These and additional objects are realized inaccordance with the present invention by passing a coherent light beamthrough a periodic diffusing structure before striking a non-diffuseobject desired to be illuminated. By periodic structure",is meant hereina diffusing structure which alters the phase of a coherent illuminatingwavefront according to a periodically varying function thereacross. Aperiodic structure has been found to have certain desirablecharacteristics for use in such a coherent illuminating system.

One of these desirable characteristics is a periodic structuresself-imaging property. By self-imaging" is meant herein thecharacteristic where recurring planes downstream from the diffusingstructure have an intensity distribution thereacross substantially thesame as the intensity distribution across the light beam immediatelyupon emerging from the diffusing structure. The degree of correspondencebetween the intensity distribution across the self-imaging planes andthe diffusing structure is dependent upon the spatial frequenciesdeveloped by the diffusing structure relative to the bandwidth of theoptical system. In most cases, the intensity distribution at aself-imaging plane does not have an exact correspondence because thediffusing structure will develop certain diffracted orders of highspatial frequency which will be slot; that is, will not be captured bythe object. However, if a periodic structure is chosen to havesubstantially uniform intensity transmission across its surface and solong as the spatial frequencies are not too high for a given opticalsystem, its self-imaging planes downstream therefrom have an intensitydistribution which is uniform enough that speckle is not introduced intoan image of an object illuminated at such planes. The self-imagingplanes of a periodic structure repeat with a period related to theperiod of the phase variation across its surface. For certainillumination applications, therefore, a periodic structure can bedesigned so the self-imaging planes are close enough together toadequately illuminate a threedimensional object space within theresolution capability desired.

However, the resolution in depth of an imaging system is often desiredto be greater than that made possible by illuminating an object withsubstantially uniform illumination only at self-imaging planes. That is,it may be desired to view an element of the object that is smaller indepth than the distance between self-imaging planes.

It has been found that the planes intermediate of the self-imagingplanes may also be used to advantage for object illumination. Theintensity distribution across each intermediate plane is given a periodby a periodic structure which is substantially equal to the fundamentalperiod of the phase function across the diffusing structure. Therefore,an object so illuminated may be imaged by sampling the object atperiodic areas having substantially uniform intensity therebetween. Thissampling may be accomplished by masking an image of the object to allowonly areas of uniform intensity to pass, and later in the optical systemremoving the masking structure from an image by low pass spatialfiltering. Alternatively, appropriate spatial filtering alone may beutilized without the aid of a mask to sample equal intensity pointsacross a plane of the object which is imaged.

According to another aspect of the present invention, it has been foundthat if the phase function across the diffusion plate is chosen in avery particular manner, the period at which equal intensity points occuracross an intermediate plane is much smaller than the phase functionperiod across the diffusing structure and thus allows sampling of animage at equal intensity points thereacross which are closer together.Such a diffusing structure causes even a finer break-up of the unwantedinterference patterns of the image at intermediate planes withoutdecreasing the fundamental period of the phase function.

In designing a diffusing structure having a phase function whichproduces an intensity distribution at intermediate planes downstreamtherefrom having a period of occurrence of equal intensity pointsthereacross less than the fundamental period of the diffusing structure,a trial and error process has been found satisfactory. Morespecifically, an example technique is for a designer to choose a certainnumber of distinct levels of relative phases to occur within each periodacross of the diffusing structure and alter their relative values untilthe period at which equal intensity points occur across an intermediateplane is a minimum possible. The resulting diffusing structure has beenreferred to as a pseudo-random" diffuser. However, it should be notedthat the distinct relative phase values within each period are notrandomly chosen but are carefully determined to decrease the period ofoccurrence of equal intensity points across intennediate planes of thediffused light beam.

The ideal coherent light beam would be one which has a random varyingphase and uniform intensity thereacross at any plane along the beam.This would allow illumination of a two or three dimensional nondiffuseobject, such as a non-diffuse transparency, without adding a speckledappearance to the object and without allowing the formation of darkinterference rings in an image thereof caused by light scattererssomewhere in the illumination beam or in the viewing system. However,this goal is presently unattainable.

Use of the term diffusing structure herein is meant to refer to astructure which alters the relative phase of various points across acoherent light beam. It is unimportant to this invention the specifictype of diffusing structure utilized. Such a structure, for instance,may include a light refracting medium or may diffract light, the resultbeing the same for the purposes of the present invention.

In the context of this description, a non-diffuse" object is one inwhich diffracted light scattered from within the object makes up a smallpercentage of the total light that is transmitted by the object.

For a more detailed understanding of the invention and for illustrationof various specific forms thereof, reference should be had to thefollowing description taken in conjunction with the accompanyingdrawings. For a general quantitative discussion of the subject matterherein, reference should be had to applicants letter published inApplied Optics, May, 1968, pages 982-983, and to applicants jointlyauthored paper appearing in Applied Optics, October, 1968, (Vol. 7, No.10) beginning at page 2,085.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 schematically illustrates anillumination system for the use of the techniques of the presentinvention.

FIG. 2 schematically illustrates certain characteristics of a diffusionstructure employed in the techniques of this invention.

FIG. 3 shows one form of a periodic diffusion structure useful in thepractice of this invention.

FIG. 3A illustrates the relative phase function across a line of thediffusing structure of FIG. 3.

FIGS. 4, 4A and 4B schematically illustrate a method of image samplingaccording to this invention.

FIG. 5 illustrates a periodic diffusion structure which may be utilizedin the practice of the present invention. FIG. 5A illustrates a typicalphase function across line of the diffusing structure of FIG. 5.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to FIG. 1, a coherentlight source 11 illuminates a three-dimensional object space 13 whichcontains object information to be viewed or imaged. For

purposes of illustration, it is assumed that a particular plane withinthe object space 13 is desired to be viewed directly or by use of animaging system 17 to form an image of the plane 15 at an image plane 19.The imaging system 17 is schematically illustrated as a simple lens butwould probably be a complex optical system, such as in the case of amicroscope, or would not include a lens element at all in the case ofcertain types of holography. The goal is to illuminate the plane 15 andany other plane within the object space 13 in a manner that a image maybe viewed at the image plane 19 without noise due to uneven intensity atthe object plane 15 or dark interference rings due to undesired lightscatterers in the system.

The coherent light source 11 is most conveniently a laser which emits ahighly collimated beam 21 with the help of appropriate optics. Withoutany diffusing structure, a light scatterer such as dust on a lens willgenerate a spherical wavefront which interferes with the plane wavefrontemitted from the laser and is viewed at the image plane as undesirableinterference rings. This is especially a problem when athree-dimensional object is utilized since planes of the object otherthan that being viewed at a given instant may serve as light scatterersand thus generate a complex pattern of dark interference fringes. Asreferred to hereinabove, a diffusing structure 23 placed in the highlyuniform wavefront beam 21 has been used to impart a random phase to thebeam portion 25 downstream. The interference between the spherical wavesproduced by the undesirable scatterers and the random phase wavefrontproduces patterns which are very fine and therefore less noticeable whenviewed in the image plane 19.

The problem of these interference rings exists only with non-diffuseobjects since if the object illuminated is diffuse, it serves as arandom scatterer of light and breaks up any undesirable darkinterference rings and a diffusing structure is not necessary. A commonnondiffuse object utilized in coherent imaging is a transparency, and isused in the description herein. However, the invention herein has equalapplicability to nondiffuse reflective objects, such as a mirror.

The problem with diffusing structures utilized before this invention isthat an object plane 15 to be viewed or imaged, although of random phasewhich breaks up the interference rings, does not have uniform intensitythereover so adds noise to the illuminated object in the form of speckleor grain. Furthermore, this speckle cannot be removed by spatialfiltering. Referring to FIG. 2, a periodic diffusing structure 27 whichproduces recurring self-imaging planes is illustrated which may be usedaccording to this invention as the diffusing structure 23 in FIG. 1. Ifthe self-imaging diffuser 27 has a uniform intensity transmission overits surface, the self-imaging planes 29, 31, 33 etc., also hassubstantially uniform intensity. If a two dimensional transparency 35 isplaced to coincide with a self-imaging plane as shown, the transparencyis viewed or imaged with substantially uniform intensity overall. Theposition and recurrence of the self-imaging planes downstream from thediffusing structure 27 is determined by its characteristics inscattering the incident coherent light into various orders 37.

Using the self-imaging planes for illumination of planar objects isuseful since they can be illuminated substantially uniformly withouthaving to be placed immediately against the diffusing structure. Athree-.

dimensional object may be moved relative to the diffuser to position aself-imaging plane at a plane of the object. desired to be viewed.However, it is generally desirable to illuminate a three-dimensionalobject in a manner so that any plane thereof may be viewed, especiallyif a hologram thereof is to be constructed. The diffusing structure 27may be designed so that the selfimaging planes recur throughout athree-dimensional object often enough that viewing the three-dimensionalobject only at these planes is within the desired resolving power of theoptical system. However, it is difficult to generate self-imaging planesclose enough together for very good resolution viewing ofthree-dimensional objects. Therefore, it is generally preferable to beable to use the illumination at planes intermediate of selfimagingplanes. It has been found that a periodic phase varying diffusingstructure 27 gives illumination with a useful intensity distributionacross intermediate planes for continuous three-dimensional objectillumination. It has also been found that the object image atintermediate planes may be sampled at areas of equal intensity toreconstruct an image of the object with a resolution which depends uponhow close the sampled equal intensity points are to each other.

A corner of the face of an appropriate periodic structure 39 is shown inFIG. 3. The area 41 will shift incident light a uniform phase amountwhile the squares 43 are designed to shift light striking them adifferent amount relative thereto. Such a diffuser may be constructed byorienting a pair of Ronchi gratings at right angles to each other on anappropriate photographic film which is then exposed, developed andbleached to a transparent state. It is preferred that the gratings beseparated slightly from the film during exposure so that the boundariesbetween the two phase areas are not so sharp as to produce high spatialfrequencies which are not passed by the system. FIG. 3A illustrates thephase function across a line of the periodic structure 39, which can beseen not to have a sudden transition between the two relative phasevalues.

Such a periodic phase diffusing structure has certain definiteadvantages. The self-imaging planes of such a structure are located atperiodic intervals downstream from the diffusing plate. The primaryadvantage is that any plane one wants to choose between the diffuserself-imaging planes will have an intensity distribution that is alsoperiodic and each plane has an intensity distribution of the sameperiod. Therefore, an image of an object may be sampled at the sameperiod across any plane thereof. The period of occurrence ofself-imaging planes downstream of the diffuser and the period of theintensity distribution across each plane intermediate thereto aredependent upon the period of the phase function across the diffusingstructure.

Referring to FIG. 4, a technique for sampling an image of anobjectilluminated by coherent light passed through the periodicdiffusing structure 39 is schematically illustrated. A coherent lightsource 45 produces a coherent light beam 47 with a highly uniformwavefront which is passed through the periodic phase diffusing structure39 to produce a diffuse coherent light beam 49 which is passed through athree-dimensional transparent object to be viewed, such as a biologicalspecimen in a microscope. An objective lens 53, preferably movable toand from the object, images (may be referred to as a first object image)a desired plane of the object transparency 51 onto a mask 55. The faceof the mask 55 is shown in FIG. 4A which includes areas of varyingopacity and transmissivity, alternating across the surface. This mask ispositioned to allow areas of the object image of uniform intensity onlyto pass, thereby throwing away other areas of the object imaged.

This masked image is then spatially filtered to remove the mask andreconstruct an image (may be referred to as the second object image) ofthe object plane viewed at the image plane 61 of FIG. 4. A lens 63 ispositioned to bring the mask image to a focus at a spatial filter 65.Since we have assumed collimated light throughout this illustration, thelens 63 will be placed its focal length ffrom the mask image and thespatial filter is placed a distance fon the other side of the lens 63.As is well-known, the intensity distribution at the spatial filter 65 isthe Fourier transform of the intensity distribution at the mask 55. Thespatial filter 65 is positioned at what is often called the frequencyplane and allows manipulation of the image which cannot be done easilyin other ways. The spatial filter 65 is shown in FIG. 4B which passeslow spatial frequencies through a transparent area 67 while blockinghigh spatial frequencies of the masked image at an opaque area 69.Another lens 71 is placed a distance equal to its focal length, whichalso may be f, on the other side of the spatial filter 65 to reconstructan image of the desired object plane absent the mask 55 at the imageplane 61 located a distance f on the opposite side of the lens 71.Alternatively, appropriate spatial filtering without use of a mask maybe employed.

The period of the phase function across such a periodic structure asillustrated in FIG. 3 determines the size of the dark segments remainingin the image from the broken-up interference rings as well asdetermining the distance between equal intensity points. The lower thisperiod (the more rapid the phase variation), the smaller are theundesirable dark interference spots and the closer are points of equalintensity across the illumination beam.

It is often desirable to reduce the size of such dark spots and toincrease the frequency of occurrence of equal intensity points acrossthe coherent beam at planes intermediate of self-imaging planes, withoutreducing the period of the phase variation across the diffusingstructure. It has been discovered that these objectives may be realizedby constructing a diffusing structure with a phase function more complexthan that of the FIG. 3 structure but one having approximately the samefundamental period thereacross.

An example of such a diffusing structure is illustrated in FIG. 5wherein six distinct phase values have been chosen for illustration,designated as 1b, 100, 4a, (in 4 41 It should be noted that therectangle of three individual phase shifting squares on the vertical bytwo squares on the horizontal comprises the basic element of thediffusing structure which is repeated periodically thereacross. FIG. 5Ashows a possible phase function across a vertical line of the FIG. 5diffusing structure. It may be noted that within a single period 1- thephase function is more complex than that within a period 1- of FIG. 3A.It should also be noted that the phase variation within a period may bea continuous function but is illustrated here to have discrete values(1),, and Having discrete phase values allows one constructing adiffuser for a particular application to manipulate these relativevalues until it is determined mathematically that points of equalintensity occur across a desired downstream plane at intervals closeenough together so that sampling such equal intensity points willconstruct an image of desired resolution. Once these relative phasevalues are determined, the diffuser may be constructed by holographictechniques and inserted into an imaging system such as that of FIG. 4 asdescribed hereinabove.

It shall be understood the invention is not limited to the specificarrangements shown, and that changes and modifications may be madewithin the scope of the appended claims.

What is claimed is:

1. A method for illuminating a non-diffuse object with coherent light,comprising the steps of:

directing a beam of coherent light toward the nondiffuse object,positioning a light diffuser in the path of said coherent light beam,said diffuser characterized by a continuous smoothly varying periodicrelative phase function and substantially uniform intensitytransmittance thereacross, thereby alone forming several planes ofsubstantially uniform intensity thereover across the diffused beam atvarious intervals along its length, said substantially uniform intensityplanes occurring only where the diffuser self-images, and

positioning the object in the path of the diffused beam a distance fromthe diffuser with at least one plane of the object coinciding with aself-imaging plane of the diffused beam having substantially uniformintensity.

2. A method of illuminating a non-diffuse object and forming an imagethereof, comprising the steps of:

directing coherent light in an illuminating beam to ward said object,

diffusing said illuminating beam prior to striking the object toilluminate some portions of the object with light of substantially equalintensity, and other portions with light of different intensity forminga first image of the illuminated object,

sampling portions of said first image corresponding to portions of theobject illuminated with light of substantially equal intensity, and

forming a second image of said object only from the sampled portions ofsaid first image, thereby to discard information of the object betweensampled areas.

3. The method according to claim 2 wherein the step of diffusing thecoherent beam includes passing said beam through a diffusing structurewhich is characterized by substantially uniform intensity transmittanceand by a periodically varying phase function thereacross.

4. The method according to claim 3 wherein the step of sampling portionsof the first image includes the step of placing a mask over said firstimage of the object to block all but the sampled portions, and whereinthe step of forming a second image includes spatially filtering the maskfrom the sampled first image of the object.

5. A method of illuminating a non-diffuse object and forming an imagethereof, comprising the steps of:

directing coherent light in an illuminating beam toward said object,

diffusing said illuminating beam across a diffusing surface prior tostriking the object by altering the relative phase across said beamaccording to a periodically varying relative phase function across saidsurface, whereby at least one plane along the diffused beam hassubstantially equal light intensity areas distributed periodicallythereacross and other areas of different light intensity,

positioning said object in the path of the diffused beam so that atleast one plane of said object is coincident with said at least oneplane of said diffused beam, and

forming a sampled image only from portions of the objects said at leastone plane that are illuminated by said substantially equal lightintensity areas of said beam.

6. The method according to claim wherein the step of diffusing said beamincludes adjusting the relative phase function period across thediffusing surface so that said substantially equal light intensity areasoccur periodically across said at least one plane of the diffused beamwith a period substantially equal to or less than an object element sizewhich is desired to be resolved in the image of the object.

7. The method according to claim 5 wherein the step of forming a sampledimage includes the steps of placing a mask over a first image of theobject to block all but areas thereof corresponding to portions of theobje ct thatare illuminated by said substantially equal light intensityareas of said beam, and spatially filtering the mask from the maskedfirst image of the object, thereby forming said sampled image of theobject.

8. A system for illuminating a non-diffuse object with coherent light,comprising,

a source of a coherent light beam,

a diffuser located in the path of said light beam, said diffusercharacterized by a continuous smoothly varying periodic relative phasefunction thereacross and a substantially uniform light transmittancethereover, thereby producing a diffused light beam with a plurality ofplanes therealong of substantially uniform intensity across the beam,said substantially uniform intensity planes ocurring only where saiddiffuser self-images, and

a non-diffuse object positioned in the path of said diffused light beamcoincident with at least one of said plurality of substantially uniformintensity planes a distance from the diffuser.

9. The improved system according to claim 8 wherein the period of saidphase varying structure is small enough so the self-imaging planesrepeat within a distance equal to the smallest object element sizedesired to be resolved in an image.

10. The improved system according to claim 8 wherein the period of saidphase varying structure is small enough so that points of substantiallyequal intensity repeat across planes intermediate of self-imaging planeswith a period equal to or less than the smallest object element sizedesired to be resolved in an image.

11. A system for viewing a non-diffuse object with coherent light,comprising:

a source of a coherent light beam,

a diffuser positioned in the path of said coherent light beam, saiddiffuser characterized by a periodic relative phase varying functionthereacross, thereby generating a diffused coherent light beamcharacterized by a plurality of diffuser self-imaging planes recurringperiodically along the duffused beam with planes intermediate of saidself-imaging planes having repetitive points thereacross ofsubstantially equal intensity and other points of different intenmy,

a non-diffuse object positioned in the path of said diffused coherentlight beam for illumination thereby, and

means for sampling the illuminated object at points thereacrosscorresponding to said points of substantially equal intensity that recuracross intermediate planes of said diffused light beam for forming anobject image from the sampled object points.

12. A coherent viewing system according to claim 11 wherein said meansfor sampling includes a mask of periodically varying light transmission.

13. A coherent viewing system according to claim 12 wherein said meansfor sampling additionally includes appropriate low pass spatialfiltering apparatus for re moving said mask from said object image.

14. A coherent viewing system according to claim 11 wherein saiddiffuser is additionally characterized by substantially uniform lighttransmittance thereover.

15. A coherent viewing system according to claim 14 wherein the periodicrelative phase varying function of said diffuser consists of at leastthree distinct levels of relative phase change in each period across thediffuser in at least one direction.

16. A method of illuminating a non-diffuse object and forming an imagethereof, comprising the steps of:

generating a coherent light beam with at least one plane therealonghaving distributed thereacross predetermined areas of substantiallyequal light intensity and other areas of different light intensity,

positioning a non-diffuse object in the path of said beam coincidentwith said at least one plane, and

forming a sampled image only from portions of said object coincidentwith said at least one light beam plane that are illuminated by saidsubstantially equal light intensity areas.

17. The method according to claim 16 wherein the step of generating acoherent light beam includes passing a light beam from a coherent lightsource through a diffuser having in at least one direction across thebeam a plurality of distinct relative phase values joined togetherthereacross in the nature of a step function as opposed to a continuoussmoothly varying relative phase function.

18. The method according to claim 17 wherein said diffuser includes atleast three distinct relative phase values across the beam in said atleast one direction.

19. The method according to claim 17 wherein said diffuser includes atleast three distinct relative phase values which form a period of phasevariation that is repeated across the diffuser in said at least onedirection.

20. A method of illuminating and imaging a nondiffuse object withcoherent light, comprising the steps of:

directing a beam of coherent light toward the nondiffuse object,

positioning a light diffuser in the path of said coherent light beam,said diffuser characterized by a continuous smoothly varying periodicrelative phase function and substantially uniform intensitytransmittance thereacross, thereby generating a diffused coherent lightbeam characterized by a plurality of diffuser self-imaging planesrecurring periodically along the diffused beam and having substantiallyuniform intensity thereacross only at said self-imaging planes,

. 1 1 4 12 positioning the non-diffuse object in the path of the imagingthe illuminated object with an optical system d'ffused beam 3 dlstancefrom the d'ffuser comm that captures substantially all spatialfrequencies dent with a self-imaging plane of the diffused beam that hassubstantially uniform intensity thereacross,

and

developed by said light diffuser.

UNl'll-. l) s'rA'nas lA'll'lN'l. omen CER'LIFICA'LE OF CORRECTION3,754,814 Dated August 28, 1973 Patent No.

it is certified that error appears in the above-identified patent andthat said Letters Patent are hereby correeted as shown below:

Column 3, line'8', change the word "slot" to --lost-.

I Column 7, line 53, ohange "100" to Column 9, line 66, correct spellingof "duffused" to -diffused--.

Signed and sealed this 18th day of December 1973.

(SEAL) Attest:

EDWARD M.PLETCHER,JR. D I RENE D. TEGTMEYER Attesting Officer ActingCommissioner of Patents 3,754 ,814 Dated August 28, 1973 Extent No.

the above-identified patent It is certified that error appears in I reated as shown below:

anfi that said Letters Patent are hereby cor Column 3, line'8', changethe word "slot" to lost-.

' ,Column 7, line 53, Lch to Column 9, line 66, cprreetspelling of"dtiffus ed" to -diffused" Signed and sealed this 18th day of December1973.

(SEA-L) Attest:

RENE D. TEGTMEYER Acting Commissioner of Patents EDWARD M.PLET.CHER,JR.Attesting Officer

1. A method for illuminating a non-diffuse object with coherent light,comprising the steps of: directing a beam of coherent light toward thenon-diffuse object, positioning a light diffuser in the path of saidcoherent light beam, said diffuser characterized by a continuoussmoothly varying periodic relative phase function and substantiallyuniform intensity transmittance thereacross, thereby alone formingseveral planes of substantially uniform intensity thereover across thediffused beam at various intervals along its length, said substantiallyuniform intensity planes occurring only where the diffuser self-images,and positioning the object in the path of the diffused beam a distancefrom the diffuser with at least one plane of the object coinciding witha self-imaging plane of the diffused beam having substantially uniformintensity.
 2. A method of illuminating a non-diffuse object and formingan image thereof, comprising the steps of: directing coherent light inan illuminating beam toward said object, diffusing said illuminatingbeam prior to striking the object to illuminate some portions of theobject with light of substantially equal intensity, and other portionswith light of different intensity forming a first image of theilluminated object, sampling portions of said first image correspondingto portions of the object illuminated with light of substantially equalintensity, and forming a second image of said object only from thesampled portions of said first image, thereby to discard information ofthe object between sampled areas.
 3. The method according to claim 2wHerein the step of diffusing the coherent beam includes passing saidbeam through a diffusing structure which is characterized bysubstantially uniform intensity transmittance and by a periodicallyvarying phase function thereacross.
 4. The method according to claim 3wherein the step of sampling portions of the first image includes thestep of placing a mask over said first image of the object to block allbut the sampled portions, and wherein the step of forming a second imageincludes spatially filtering the mask from the sampled first image ofthe object.
 5. A method of illuminating a non-diffuse object and formingan image thereof, comprising the steps of: directing coherent light inan illuminating beam toward said object, diffusing said illuminatingbeam across a diffusing surface prior to striking the object by alteringthe relative phase across said beam according to a periodically varyingrelative phase function across said surface, whereby at least one planealong the diffused beam has substantially equal light intensity areasdistributed periodically thereacross and other areas of different lightintensity, positioning said object in the path of the diffused beam sothat at least one plane of said object is coincident with said at leastone plane of said diffused beam, and forming a sampled image only fromportions of the object''s said at least one plane that are illuminatedby said substantially equal light intensity areas of said beam.
 6. Themethod according to claim 5 wherein the step of diffusing said beamincludes adjusting the relative phase function period across thediffusing surface so that said substantially equal light intensity areasoccur periodically across said at least one plane of the diffused beamwith a period substantially equal to or less than an object element sizewhich is desired to be resolved in the image of the object.
 7. Themethod according to claim 5 wherein the step of forming a sampled imageincludes the steps of placing a mask over a first image of the object toblock all but areas thereof corresponding to portions of the object thatare illuminated by said substantially equal light intensity areas ofsaid beam, and spatially filtering the mask from the masked first imageof the object, thereby forming said sampled image of the object.
 8. Asystem for illuminating a non-diffuse object with coherent light,comprising, a source of a coherent light beam, a diffuser located in thepath of said light beam, said diffuser characterized by a continuoussmoothly varying periodic relative phase function thereacross and asubstantially uniform light transmittance thereover, thereby producing adiffused light beam with a plurality of planes therealong ofsubstantially uniform intensity across the beam, said substantiallyuniform intensity planes ocurring only where said diffuser self-images,and a non-diffuse object positioned in the path of said diffused lightbeam coincident with at least one of said plurality of substantiallyuniform intensity planes a distance from the diffuser.
 9. The improvedsystem according to claim 8 wherein the period of said phase varyingstructure is small enough so the self-imaging planes repeat within adistance equal to the smallest object element size desired to beresolved in an image.
 10. The improved system according to claim 8wherein the period of said phase varying structure is small enough sothat points of substantially equal intensity repeat across planesintermediate of self-imaging planes with a period equal to or less thanthe smallest object element size desired to be resolved in an image. 11.A system for viewing a non-diffuse object with coherent light,comprising: a source of a coherent light beam, a diffuser positioned inthe path of said coherent light beam, said diffuser characterized by aperiodic relative phase varying function thereacross, thereby generatinga diffused coherent light beam characterized bY a plurality of diffuserself-imaging planes recurring periodically along the duffused beam withplanes intermediate of said self-imaging planes having repetitive pointsthereacross of substantially equal intensity and other points ofdifferent intensity, a non-diffuse object positioned in the path of saiddiffused coherent light beam for illumination thereby, and means forsampling the illuminated object at points thereacross corresponding tosaid points of substantially equal intensity that recur acrossintermediate planes of said diffused light beam for forming an objectimage from the sampled object points.
 12. A coherent viewing systemaccording to claim 11 wherein said means for sampling includes a mask ofperiodically varying light transmission.
 13. A coherent viewing systemaccording to claim 12 wherein said means for sampling additionallyincludes appropriate low pass spatial filtering apparatus for removingsaid mask from said object image.
 14. A coherent viewing systemaccording to claim 11 wherein said diffuser is additionallycharacterized by substantially uniform light transmittance thereover.15. A coherent viewing system according to claim 14 wherein the periodicrelative phase varying function of said diffuser consists of at leastthree distinct levels of relative phase change in each period across thediffuser in at least one direction.
 16. A method of illuminating anon-diffuse object and forming an image thereof, comprising the stepsof: generating a coherent light beam with at least one plane therealonghaving distributed thereacross predetermined areas of substantiallyequal light intensity and other areas of different light intensity,positioning a non-diffuse object in the path of said beam coincidentwith said at least one plane, and forming a sampled image only fromportions of said object coincident with said at least one light beamplane that are illuminated by said substantially equal light intensityareas.
 17. The method according to claim 16 wherein the step ofgenerating a coherent light beam includes passing a light beam from acoherent light source through a diffuser having in at least onedirection across the beam a plurality of distinct relative phase valuesjoined together thereacross in the nature of a step function as opposedto a continuous smoothly varying relative phase function.
 18. The methodaccording to claim 17 wherein said diffuser includes at least threedistinct relative phase values across the beam in said at least onedirection.
 19. The method according to claim 17 wherein said diffuserincludes at least three distinct relative phase values which form aperiod of phase variation that is repeated across the diffuser in saidat least one direction.
 20. A method of illuminating and imaging anon-diffuse object with coherent light, comprising the steps of:directing a beam of coherent light toward the non-diffuse object,positioning a light diffuser in the path of said coherent light beam,said diffuser characterized by a continuous smoothly varying periodicrelative phase function and substantially uniform intensitytransmittance thereacross, thereby generating a diffused coherent lightbeam characterized by a plurality of diffuser self-imaging planesrecurring periodically along the diffused beam and having substantiallyuniform intensity thereacross only at said self-imaging planes,positioning the non-diffuse object in the path of the diffused beam adistance from the diffuser coincident with a self-imaging plane of thediffused beam that has substantially uniform intensity thereacross, andimaging the illuminated object with an optical system that capturessubstantially all spatial frequencies developed by said light diffuser.